Flexible pipes for transporting liquids that are reinforced with braided fibers are known and are used for such varied purposes as fire hoses, hydraulic hoses and irrigation pipes. The pipes generally have a multilayer construction in which a preformed pipe body comprising a flexible substrate material such as a suitable polymer, is bonded to at least one layer of fabric or layer comprising a mesh of natural or artificial fibers. The fibers typically comprise at least two sets of fibers with a first set of fibers oriented at a first angle with respect to the pipe axis and crisscrossing over a second set of fibers oriented at a second angle with respect to the pipe axis. The angle at which a set of fibers is oriented with respect to the pipe axis is conventionally referred to as a “bias angle”.
The reinforcing fibers provide the pipes with enhanced tear resistance, durability and generally improved structural integrity and resistance to stress generated by pressure of the liquids the pipes are used to transport. Stresses generated by pressure of liquid on a wall containing the liquid are longitudinal stress, hoop stress and radial stress. Longitudinal stress is parallel to an axis along the length of a pipe and typically causes a pipe to stretch and lengthen. Hoop stress is stress that operates to stretch material in the pipe wall circumferentially relative to the pipe axis. Radial stress operates radially to push the pipe wall outward from the axis.
The number and types of layers and/or sets of fibers in a given pipe and their respective bias angles are configured to provide the pipe with desired ability to withstand the various stresses to which the pipe is expected be subjected in its intended use. For some applications, for example for fire hoses, one set of fibers often has a bias angle equal to about 0° with fibers running lengthwise along the pipe, parallel to the pipe axis. A second set of fibers often has a 90° bias angle with fibers oriented substantially perpendicular to the pipe axis. U.S. Pat. No. 6,217,975 describes a ribbon of material, “extendible sheet member”, comprising fibers which generate stresses in the material that cause the ribbon to curl up to form a pipe when the ribbon is unwound from a reel. The patent asserts that the technology for producing the material could be put to use for “cable conduits, water pipes, sewage pipes, drainage and irrigation pipes, . . . ”.
For some pipes it is desired that change in length of the pipe generated by changes in pressure of liquid in the pipe be minimized. For such pipes, reinforcing fibers are typically oriented at a bias angle, conventionally referred to as a “neutral bias angle”, having magnitude equal to about 54.7°. Typically, the pipe comprises at least one set of fibers oriented at a positive neutral bias angle equal to +54.7° with respect to the pipe axis that crisscrosses over at least one second set of fibers oriented at a negative neutral bias angle equal to −54.7° with respect to the pipe axis. The two fibers in the at least one first set of fibers and at least one second set of fibers crossover at a “crossover” angle of 109.4°.
Whereas any strain in a pipe resulting from liquid pressure stress can be problematic in a given application, for long flexible pipes that are often used for agricultural irrigation, longitudinal stress and resultant longitudinal strain, i.e. elongation, can be particularly bothersome. A total longitudinal strain by which a pipe elongates when subject to a given longitudinal stress is proportional to the length of the pipe. The longer the pipe, the more a total amount by which it lengthens for a given magnitude of longitudinal stress.
Irrigation pipes typically have irrigation accessories such as emitters and/or various fittings in or attached to the pipes, for releasing water from the pipes to plants for which the pipe is used to provide irrigation or for distributing the water to other irrigation pipes, conventionally referred to as “laterals”. The pipes and emitters are generally carefully positioned relative to the plants to which the pipes provide water and emission of water via the emitters is generally carefully controlled. For flexible agricultural pipes, lengthening can cause the pipe to “snake” or curl and hamper water flow from the irrigation accessories as well as substantially to displace the irrigation accessories from their intended positions.